Typically, the objects are products such as manufactured articles or packaged food stuffs and the printer is used to print product and batch information, “use by” dates etc. The printer may be a non-impact printer such as an ink jet printers or a laser marker (i.e. lasers that print by directing a laser beam at an object to be printed onto so as to mark the object by changing a surface characteristic of the object). The printer may be a continuous ink jet printer, for example an electrostatic deflection continuous ink jet printer.
In order to position the printing correctly on each object, it is known to use a sensor upstream of the printer to detect an approaching object and trigger printing. In order to position the printing correctly, the system also needs to delay the start of printing, following detection of the approaching object, by the time it takes the object to travel the distance from its position when it is detected to its position for the start of printing. It is known to calculate this delay from the distance to be traveled (which is known) and the line speed (i.e. the speed at which objects are carried past the printer by a conveyor). The line speed may also be used to adjust the printing operation to ensure the correct spacing of the printing in the direction of movement of the objects and to adjust other factors that control print quality. The line speed may be detected using a shaft encoder, or alternatively a second sensor may be used, spaced from the first sensor in the direction of travel of the objects, and the line speed can be calculated from the time taken for an object to travel from one sensor to the other. The sensor or sensors typically comprise a photocell. For example, each sensor may be constructed as a light source and a photodetector positioned close together, so that the photodetector detects light originating from the light source and reflected by an object when the object is present.
In order for the sensors to detect the presence of an object reliably, they must be able to distinguish between the signal received when an object is present and the signal received in the absence of any object. It is common for the sensor to be arranged so that it faces the conveyor surface, with the consequence that the signal detected in the absence of any object depends on the amount of light reflected by the conveyor. This can vary considerably, depending on the design of the conveyor. Additionally, in the case of a traditional belt-type conveyor there is normally a position where the ends of the belts were joined together to make a continuous belt, and the material at the join may be more reflective than the surface of the remainder of the belt.
It is known to provide sensors with an adjustable sensitivity, and a sensor may have a “learn” button for use by an operator to control an operation in which the sensor adjusts its sensitivity. By pressing the “learn” button in a particular way, the operator may cause the sensor to take a reading of the signal received by the photodetector and use it to set a base level representing the signal received when only the visual background is present. By pressing the “learn” button in a different way, the operator may cause the sensor to take a reading of the signal received by the photodetector and use it to set a detection level representing the signal received when a detection signal is required. The sensor then sets its sensitivity to a level between the base level and the detection level. Provided that the operator uses this button correctly, presents the visual background to the sensor for the base level reading and presents a sample object to the sensor for the detection level reading, the sensor can be set to distinguish reliably between the background and the objects.